1. Field of the Invention
The present invention relates to a method of epitaxially growing compound semiconductor on a silicon wafer.
2. Description of the Related Art
With a conventional method of epitaxially growing compound semiconductor such as two-element semiconductor on a wafer made of single-element such as Si, there are formed a defect called an anti-phase domain because different atoms are arranged on the same plane and a number of crystal defects called dislocation caused by a difference between lattice constants and thermal expansion factors inherent to heteroepitaxy. In order to solve such problems, a method called a two-step growth method is generally used in growing compound semiconductor, which is disclosed in JP-A-61-26216 and JP-A-61-70715. FIG. 7 illustrates a supply of source materials and a temperature change of a susceptor according to the two-step growth method. For the comparison with FIG. 7, a supply of source materials and a temperature change of a susceptor according to homoepitaxy used for forming a compound semiconductor is shown in FIG. 6. A conventional two-step growth method will be described with reference to FIGS. 7, 8A, and 8B. First, a silicon wafer is cleaned which has the surface inclined by 2 to 5 degrees in the direction &lt;1 1 0&gt; from the plane (0 0 1). The surface oxide film of the wafer is first exposed to hydrofluoric acid to etch it, and then subjected to a thermal treatment under a hydrogen atmosphere at a temperature of 800.degree. C. or higher within a compound semiconductor manufacturing apparatus to completely remove the surface oxide film (Step (1)). Next, after the temperature is lowered to about 450.degree. C., source material gasses are introduced into the apparatus to deposit amorphous compound semiconductor to a thickness of 20 to 200 nm (Step (2)). Thereafter, the temperature is raised to about 650.degree. to 700.degree. C. (Step (3)). During this temperature raising period, the amorphous compound semiconductor changes to a single crystal layer. By using the single crystal layer as a buffer layer, compound semiconductor is grown on it (Step (4)).
Specifically, in growing GaAs compound semiconductor, a thin amorphous layer is first formed (FIG. 8A) which is re-crystallized during the temperature raising period to form a buffer layer of a poor surface flatness (FIG. 8B). Since a growth layer is formed on such a buffer layer, from the viewpoint of surface morphology, the growth layer has a waved surface characteristic of GaAs/Si without reflecting the flatness of the silicon wafer. This uneven surface adversely affects the distribution of threshold values of LSIs. A report regarding this issue was presented at the Spring Meeting of the Japan Society of Applied Physics, 1992. This report teaches that the surface flatness can be improved if the growth is temporarily terminated under the uneven surface condition, and it is resumed after the surface is polished to the degree of the same flatness as the silicon wafer.
The first problem associated with the above-described conventional technique is that the productivity is very low as compared to the homoepitaxy illustrated in FIG. 6, because the second-step growth method requires the four steps, including a step of cleaning the surface of a wafer, a step of setting a temperature suitable for the deposition of an amorphous layer, a step of raising the temperature suitable for crystallizing the amorphous layer into a buffer layer, and a step of setting the temperature suitable for growing compound semiconductor. The second problem resides in the verification that because the amorphous layer is as thin as 200 nm or less, the single crystal layer crystallized during the temperature raising period is not two-dimensionally continuous but is formed by many islands and does not cover the whole surface of the silicon wafer (The Sumitomo Search No. 47, October 1991, Applied Physic Letter 59 (26), 23 Dec. 1991).
As above, from the viewpoint of surface morphology, the surface of an epitaxial film grown by the two-step growth method is rough. With this conventional technique, the root mean square roughness is 4 to 5 nm even in an excellent case according to the evaluation by AFM (Atomic Force Microscopy). This conventional technique can be therefore said still not practical when considering a roughness of 3 nm at a minimum required for an LSI level. Although the re-growth method after polishing which attains an improved surface flatness of an LSI level, has been tried (GaAs and Related Compounds, 1992, Karuizawa, Inst. Phys. Ser. No. 129, Chapter 3, p. 175), the number of necessary manufacturing steps increases considerably, the productivity lowers, and the manufacturing cost becomes high. It has been long desired therefore to provide an as-grown wafer to have a surface flatness of about 3 nm comparable with an LSI level.
The surface roughness will further be detailed. In growing a single crystal compound semiconductor of a single domain on a silicon wafer by a conventional two-step growth method, a buffer layer of compound semiconductor as thin as 200 nm or less changes, during the temperature raising period (Step 2) shown in FIG. 7, from a thin amorphous continuous film shown in FIG. 8A to re-crystallized as single crystal compound semiconductor of single domain islands shown in FIG. 8B. However, the density of this single crystal buffer layer is more coarse than an ordinary single crystal. Furthermore, because only the small amount of source materials corresponding to the thin amorphous buffer layer is used and because re-crystallization starts from the location having a small growth energy, nuclei grow large at atomic level steps called a kink in the direction &lt;0 0 1&gt; dominant over other directions on the silicon wafer surface. In other words, a two-dimensional lateral growth improving the flatness is difficult so that the finished buffer layer cannot fully cover the whole surface of the silicon wafer. Compound semiconductor grown on the buffer layer reflects the inferior flatness of the layer, and other layers grown on the compound semiconductor layer also reflect it. If the buffer layer is made thicker in order to replenish source materials, rearrangement of crystals is hindered much, forming whity single crystal layer with an anti-phase domain instead of a layer with a single domain. From the above reasons, the conventional growth technique is not suitable for growing compound semiconductor on a silicon wafer having a surface flatness comparable with LSI level electronic devices.